Fundamental And Concepts Of Failure Mode and Effects Analysis (FMEA)
Failure Mode and Effects Analysis (FMEA) is a method designed to identify and fully understand potential failure modes and their causes, and the effects of failure on the system or end users, for a given product or process.It helps us to assess the risk associated with the identified failure modes, effects and causes, and prioritize issues for corrective action. Using FMEA we can identify and carry out corrective actions to address the most serious concerns. A FMEA is an engineering analysis done by a cross-functional team of subject matter experts that thoroughly analyzes product designs or manufacturing processes early in the product development process, so as to find and correct weaknesses before the product gets into the hands of the customer. An FMEA should be the guide to the development of a complete set of actions that will reduce risk associated with the system, subsystem, and component or manufacturing/assembly process to an acceptable level. If FMEA is effectively used throughout the product life cycle, it will result in significant improvements to reliability, safety, quality, delivery, and cost. Just performing an FMEA just to fill a checkbox in the Product Development Process and then filing it away, never to be seen again, is a waste of time and adds no value.As a tool in risk evaluation, FMEA is considered to be a method to identify severity of potential effects of failure and to provide an input to mitigating measures to reduce risk. In many applications, FMEA also includes an estimation of the probability of occurrence of the causes of failure and their resultant failure modes. This broadens the analysis by providing a measure of the failure mode ‘ s likelihood. To minimize risk, the likelihood of failure occurrence is reduced which increases product or process reliability. FMEA is a tool that is instrumental in reliability improvement. There are three basic cases for which FMEA process is to be applied, each with a different scope or focus:
Case.1: New designs, new technology, or new process. The scope of the FMEA is the complete design, technology, or process.
Case 2: Modifications to existing design or process.The scope of the FMEA should focus on the modification to design or process, possible interaction due to the modification and field history and can include changes in regulatory requirements.
Case 3: Use of an existing design or process in a new environment, location, application, or usage profile (including duty cycle, regulatory requirements, etc.).
The primary objective of an FMEA is to improve the design.For System FMEAs, the objective is to improve the design of the system. For Design FMEAs, the objective is to improve the design of the subsystem or component. For Process FMEAs, the objective is to improve the design of the manufacturing process. The other objectives for doing FMEAs are to identify and prevent safety hazards, minimize loss of product performance or performance degradation, improve test and verification plans (in the case of System or Design FMEAs), improve Process Control Plans (in the case of Process FMEAs), consider changes to the product design or manufacturing process, identify significant product or process characteristics, develop Preventive Maintenance plans for in-service machinery and equipment and develop online diagnostic techniques.
The three most common types of FMEAs are:
- System FMEA
- Design FMEA
- Process FMEA
It is analysis is highest-level of an entire system, made up of various subsystems. The focus is on system-related deficiencies, including system safety and system integration, interfaces between subsystems or with other systems, interactions between subsystems or with the surrounding environment, single-point failures (where a single component failure can result in complete failure of the entire system),The focus of FMEA is also on functions and relationships that are unique to the system as a whole (i.e., do not exist at lower levels) and could cause the overall system not to work as intended, human interactions and service. Some practitioners separate out human interaction and service into their own respective FMEAs.
Analysis is at the subsystem level (made up of various components) or component level. The Focus is on product design-related deficiencies, with emphasis on improving the design, ensuring product operation is safe & reliable during the useful life of the equipment and interfaces between adjacent components. Design FMEA usually assumes the product will be manufactured according to specifications.
Analysis is at the manufacturing/assembly process level. The Focus is on manufacturing related deficiencies, with emphasis on improving the manufacturing process and to ensure that the the product is built to design requirements in a safe manner, with minimal downtime, scrap and rework. Process FMEA also emphasis on manufacturing and assembly operations, shipping, incoming parts, transporting of materials, storage, conveyors, tool maintenance, and labeling. Process FMEAs most often assume the design is sound.
The common elements of FMEA are:
- Identify the team:
The development of FMEA should be the responsibility of cross functional or multi disciplinary team, whose members should have the necessary subject matter knowledge which also includes the knowledge of FMEA process. The team leader should have the necessary facilitation expertise and should select team member with relevant expertise and necessary authority. The team approach benefits the FMEA development process and ensure input & collaboration from all affected function areas.
- Define the scope:
ScopeestablishestheboundaryofFMEA analysis. Scope is essential because itsetslimitsonagivenFMEA, that is, it makes it finite. It defines what is included and excluded, determined based on the type of FMEA being developed, i.e., system, subsystem, or component. BeforetheFMEA can begin, a clear understanding of What is to be evaluated must be determined. What to exclude can be just as important as what to include in the analysis. The scope needs to be established at the start of the process to assure consistent direction and focus. Several documents may assist the team in determining the scope of a Process FMEA such as Function Model, Block (Boundary) diagrams, Parameter (P) diagrams, Interface diagram, Processﬂow diagram, Interrelationship matrices, Bill of Materials(BOM),
- System FMEA:
A System is made up of Various subsystems. Examples of systems include Chassis System, Powertrain System, or Interior System,etc. The focus of the System FMEA is to address all interfaces and interactions among systems, subsystems, the environment and the customers
- Subsystem FMEA:
A Subsystem FMEA is a subset of a System FMEA. An example of a subsystem is the front suspension subsystem, which is a subset of the chassis system. The focus of the Subsystem FMEA is to address all interfaces and interactions among the subsystem components and interactions with other subsystems or systems.
- Component FMEA:
A Component FMEA is a subset of a subsystem FMEA. For example, a brake pad is a component of the brake assembly, which is a subsystem of the chassis system.
NOTE: Any subsequent adjustments to the scope may require a modification of the tearn structure and membership.
- System FMEA:
- Define the Customer:
Customer knowledge can contribute precise definition of functions, requirements, and specifications.Knowledge of these customers can help to define the functions,requirementsandspeciﬁcations more robustly as well as aid in determining effects of related failure modes. For examples OEM will have four major customerstobeconsideredintheFMEA process,
- End Users:
the person or organization that will utilize product. The FMEA analysis affecting the End User could include, for example, durability.
- OEM ASSEMBLY and MANUFACTURING CENTERS (PLANTS):
the OEM locations where manufacturing operations (e.g., stamping and powertrain) and vehicle assembly take place. Addressing the interfaces between the product and its assembly process is critical to an effective FMEA analysis.
- Supply Chain Manufacturing:
The supplier location where manufacturing, fabricating or assembling of production materials or parts takes place. This includes fabricating production and service parts and assemblies and processes such as heat treating; welding, painting,plating or other ﬁnishing services;‘These.may be any subsequent or downstream operation or next tier manufacturing process.
Government agencies that define requirements and monitor compliance to safety and environmental specifications which can impact the product or process.
- End Users:
- Identify Functions, Requirements, Specifications:
Identify and understand the functions, requirements and speciﬁcations relevant to the deﬁned scope. The purpose of this activity is to clarify the item design intent or process purpose. This assists in the determination of the potential failure mode for each attribute or aspect of the function.
- Identify Failure Modes:
Failure mode is deﬁned as the way or manner in which a product or process could fail to meet design intent or process requirements. The assumption is made that the failure could occur but may not necessarily occur. A concise and understandable failure definition is important since it properly focuses the analysis. Failure modes should be described in technical terms and not as a symptom necessarily noticeable by the customer. A large number of failure modes identified for a single requirement may indicate that the deﬁned requirement is not concise.
- Identify Effects:
Potential effects of failure are deﬁned as the effects of the failure mode as perceived by the customer. The effects or impact of the failure are described in terms of what the customer might notice or experience. The customer may be an internal customer as well as the End User. Determining potential effects includes the analysis of the consequences of the failures and the severity or seriousness of those consequences.
- Identify Potential Causes:
Potential cause of failure is deﬁned as an indication of how the failure could occur, described in terms of something that can be corrected or can be controlled. Potential cause of failure may be an indication of a design Weakness, the consequence of which is the failure mode. There is a direct relation between a cause and its resultant failure mode (i.e., if the cause occurs, then the failure mode occurs). Identifying the root cause(s) of the failure mode, in sufficient detail, enables the identification of appropriate controls and action plans. A separate potential cause analysis is performed for each cause if there are multiple causes.
- Identify Controls:
Controls are those activities that prevent or detect the cause of the failure or failure mode. In developing controls it is important to identify what is going wrong, why, and how to prevent or detect it. Controls are applicable to product design or manufacturing processes. Controls focused on prevention will provide the greatest return.
- Identifying and Assessing Risks:
One of the important steps in the FMEA process is the assessment of risk. This is evaluated in three ways i.e by severity, occurrence, and detection. Severity is an assessment of the level of impact of a failure on the customer.
Occurrence is how often the cause of a failure may occur. Detection is an assessment of how well the product or process controls detect the cause of the failure or the failure mode. Organizations need to understand their customer requirements for risk assessment.
- Recommended Actions and Results:
The intent of recommended actions is to reduce overall risk and likelihood that the failure mode will occur. The recommended actions address reduction of the severity, occurrence and detection.
The following can be used to assure that the appropriate actions are taken, including but not limited to:
- Ensuring design requirements including reliability are achieved,
- Reviewing engineering drawings and speciﬁcations,
- Conﬁrming incorporation in assembly/manufacturing processes, and,
- Reviewing related FMEAS, control plans and operations instructions.
Responsibility and timing to complete the recommended actions should be recorded. Once actions are completed and results captured, the updated ratings for severity, occurrence and detection should also be recorded.
- Management Responsibility:
Management owns the FMEA process. Management has the ultimate responsibility of selecting and applying resources and ensuring an effective risk management process including timing. Management responsibility also includes providing direct support to the team through on-going reviews, eliminating roadblocks, and incorporating lessons learned.
An “item” is the focus of the FMEA project. For a System FMEA this is the system itself. For a Design FMEA, this is the subsystem or component under analysis.For a Process FMEA, this is usually one of the specific steps of the manufacturing or assembly process under analysis, as represented by an operation description.
Example: Bicycle hand brake subsystem
A “function” is what the item or process is intended to do, usually to a given standard of performance or requirement.
For Design FMEAs, this is the primary purpose or design intent of the item.For Process FMEAs, this is the primary purpose of the manufacturing or assembly operation.Functions are typically described in a verb-noun format.There can be many functions for each item or operation.
Example: Provides the correct level of friction between brake pad assembly and wheel rim to safely stop bicycle in the required distance, under all operating conditions.
- Failure Mode:
The term “failure mode” combines two words that both have unique meanings.The Concise Oxford English Dictionary defines the word “failure” as the act of ceasing to function or the state of not functioning.“Mode” is defined as a way in which something occurs.A “failure mode” is the manner in which the item or operation potentially fails to meet or deliver the intended function and associated requirements.It may include failure to perform a function within defined limits, inadequate or poor performance of the function,intermittent performance of a function and/or performing an unintended or undesired function.
Example: Insufficient friction delivered by hand brake subsystem between brake pads and wheels during heavy rain conditions.
An “effect” is the consequence of the failure on the system or end user.This can be a single description of the effect on the top-level system and/or end user, or three levels of effects (local, next-higher level, and end effect)For Process FMEAs, consider the effect at the manuf. or assembly level, as well as at the system or end user.There can be more than one effect for each failure mode. However, typically the FMEA team will use the most serious of the end effects for the analysis.
Example: Bicycle wheel does not slow down when the brake lever is pulled potentially resulting in accident.
“Severity” is a ranking number associated with the most serious effect for a given failure mode. It is based on the criteria from a severity scale. It is a relative ranking within the scope of the specific FMEA. It is determined without regard to the likelihood of occurrence or detection.
A “cause” is the specific reason for the failure, preferably found by asking “why” until the root cause is determined.For Design FMEAs, the cause is the design deficiency that results in the failure mode.For Process FMEAs, the cause is the manufacturing or assembly deficiency that results in the failure mode.At the component level, cause should be taken to the level of failure mechanism.If a cause occurs, the corresponding failure mode occurs. There can be many causes for each failure mode.
Example: Cable breaks
“Occurrence” is a ranking number associated with the likelihood that the failure mode and its associated cause will be present in the item being analyzed.For System and Design FMEAs, consider the likelihood of occurrence during the design life of the product. For Process FMEAs consider the likelihood of occurrence during production. It is based on the criteria from the corresponding occurrence scale. Ithas a relative meaning rather than absolute value, determined without regard to the severity or likelihood of detection.
“Controls” are the methods or actions currently planned, or are already in place, to reduce or eliminate the risk associated with each potential cause. Controls can be the methods to prevent or detect the cause during product development, or actions to detect a problem during service before it becomes catastrophic. There can be many controls for each cause.
- Prevention-type Controls:
For System or Design FMEAs, prevention-type design controls describe how a cause, failure mode, or effect in the product design is prevented based on current or planned actions. They are intended to reduce the likelihood that the problem will occur, and are used as input to the occurrence ranking.
Example: Cable material selection based on ANSI #ABC.
- Detection-type Controls:
For System or Design FMEAs, detection-type designs controls describe how a failure mode or cause in the product design is detected, based on current or planned actions before the product design is released to production, and are used as input to the detection ranking.They are intended to increase the likelihood that the problem will be detected before it reaches the end user.
Example: Bicycle system durability test # 789
- Prevention-type Controls:
“Detection” is a ranking number associated with the best control from the list of detection-type controls, based on the criteria from the detection scale. It considers the likelihood of detection of the failure mode/cause, according to defined criteria.It is a relative ranking within the scope of the specific FMEA. It is determined without regard to the severity or likelihood of occurrence.
- Risk Priority Number (RPN):
“RPN” is a numerical ranking of the risk of each potential failure mode/cause, made up of the arithmetic product of the three elements:
- severity of the effect
- likelihood of occurrence of the cause
- likelihood of detection of the cause.
Example: 240 (10 x 6 x 4).
RPN is not a perfect representation of the risk associated with a failure mode & associated cause as it is subjective and not continuous. High severity must be considered regardless of RPN value
- Recommended Actions:
“Recommended actions” are the tasks recommended by the FMEA team to reduce or eliminate the risk associated with potential causes of failure. They should consider the existing controls and the relative importance (prioritization) of the issue. The cost and effectiveness of the corrective action. There can be many recommended actions for each cause.
Example: Require cable DFMEA/PFMEA from cable supplier approved by All-Terrain FMEA team.
- Actions Taken:
“Action Taken” is the specific action that is implemented to reduce risk to an acceptable level.It should correlate to the specific recommended action and is assessed as to effectiveness by a revised severity, occurrence, detection ranking, and corresponding revised RPN.
Example: Cable supplier completed DFMEA/PFMEA and approved by All-Terrain team.
Steps involved in implementing FMEA:
Failure Mode and Effects Analysis (FMEA) is used to identify specific ways in which a product, process, or service might fail and to then develop countermeasures targeted at those specific failures. This will improve performance, quality, reliability, and safety. FMEA is most commonly used in the Improve step of the DMAIC method to improve the effectiveness of a proposed solution, but it is also helpful in the Recognize step for identifying improvement opportunities, and in the Measure step for determining what data to collect and where to collect it. FMEA follows the steps of the process and identifies where problems might occur. It scores potential problems based on their probability of occurrence, severity, and ability to be detected. Based on the scores mentioned above, helps to determine where countermeasures are necessary to avoid problems. IT allows re-scoring of the problem after you have put countermeasures in place.
- List the process steps in the first column of a FMEA chart like the one below.
- For each process step, brainstorm potential failure modes—ways in which the product, service, or process might fail (e.g., jams, sputters, freezes or slows up, is unreadable).
- Identify the potential consequences or effects of each failure (e.g., defective product, wrong information, delays) and rate their severity.
- Identify causes of the effects and rate their likelihood of occurrence.
- Rate your ability to detect each failure mode (in the Detection column).There might be multiple failures for each step and multiple effects for each failure. Score each separately.
- Multiply the three numbers (severity, occurrence, and detection) together to determine the risk of each failure mode. This is represented in the chart by a risk priority number, or RPN.
RPN = severity × occurrence × detection.
- Identify ways to reduce or eliminate risk associated with high RPNs.
- Re-score those failures after you put countermeasures in place.
Design Failure Mode Effect Analysis (DFMEA)
The Design Failure Mode Effects Analysis (DFMEA), supports the design process in reducing the risk of failures by aiding in the objective evaluation of the design, including functional requirements & design alternatives and by evaluating the initial design for manufacturing, assembly,service, and recycling requirements. It increases the probability that potential failure modes and their effects on system have been considered in the design/development process. It provides additional information to aid in the planning of thorough and efficient design, development, and validation programs.It helps in developing a ranked list of potential failure modes according to their effect on the customer, thus establishing a priority system for design improvements, development, and validation testing/analysis. It provides an open issue format for recommending & tracking risk-reducing actions, and providing future reference, (e.g., lessons learned), to aid in addressing ﬁeld concerns, evaluating design changes, and developing advanced designs.
The DFMEA is a living document and should be initiated before design concept finalization. It should be updated as changes occur or additional information is obtained throughout the phases of product development and be fundamentally completed before the production design is released. It should be a source of lessons learned for future design iterations.
|Example of Design Failure Mode and Effect Analysis|
Maintaining Design Failure Mode Effect Analysis (DFMEA)
The DFMEA is a living document and should be reviewed whenever there is a product design change and updated, as required. Recommended actions updates should be included into in subsequent DFMEA along with the ﬁnal results (what worked and what did not work). Another element of on-going maintenance of DFMEAs should include in periodic review of the rankings used in the DFMEA. Speciﬁc focus should be given to Occurrence and Detection rankings. This is particularly important where improvements have been made either through product changes or improvements in design controls. Additionally, in cases where ﬁeld issues have occurred, the rankings should be revised accordingly.
Leveraging Design Failure Mode Effect Analysis (DFMEA)
If a new project or application is functionally similar to the existing product, a single DFMEA may be used with customer concurrence. Using a fundamentally sound baseline DFMEA as the starting point provides the greatest opportunity to leverage past experience and knowledge. If there are slight differences, the team should identify and focus on the efforts of these differences.
Linkage of Design Failure Mode Effect Analysis (DFMEA)
The DFMEA is not a “stand-alone” document. For example, the output of the DFMEA can be used as input for subsequent product development processes. It is the summary of the team’s discussions and analysis.
Design Verification plan & Report(DVP&R) and Design Failure Mode Effect Analysis(DFMEA) have an important linkage. The DFMEA identiﬁes and documents the current design prevention and detection controls which become input to the test description included within the DVP&R. The DFMEA identiﬁes “what” the controls are while the DVP&R provides the “how” such as acceptance criteria, procedure and sample size. Another imponam linkage is between the DFMEA and PFMEA. For example a Process (PFMEA) failure mode or a Design (DFMEA) failure mode can result in the same potential product effect. In this case, the effects of the design failure mode should be reflected in the effects and severity rankings of the DFMEA and PFMEA.
Process Failure Mode Effect Analysis (PFMEA):
The process Failure Mode Effect Analysis, referred to as PFMEA, supports manufacturing process development in reducing the risk of failures by identifying and evaluating the process functions and requirements. It helps in identifying and evaluating potential product and process- related failure modes, and the effects of the potential failures on the process and customers. It identifies the potential manufacturing or assembly process causes, and process variables on which to focus process controls for occurrence reduction or increased detection of the failure conditions. It enables the establishment of is priority system for preventive/corrective action and controls.The PFMEA is a living document and should be initiated before or at the feasibility stage, prior to tooling for production. It should take into account all manufacturing operations from individual components to assemblies, and should include all processes within the plant that can impact the manufacturing and assembly operations, such as shipping, receiving, transporting of material, storage, conveyors or labeling.
Early review and analysis of new or revised processes is advised to anticipate, resolve, or monitor potential process concerns during the manufacturing planning stages of a new model or component program. The PFMEA assumes the product as designed will meet the design intent. Potential failure modes that can occur because of a design weakness may be included in a PFMEA. Their effect and avoidance is covered by the Design FMEA. The PFMEA does not rely on product design changes to overcome limitations in the process. However, it does take into consideration a product’s design characteristics relative to the planned manufacturing or assembly process to assure that, to the extent possible, the resultant product meets customer needs and expectations. For example the PFMEA development generally assumes that the machines and equipment will meet their design intent and therefore are excluded from the scope. Control mechanisms for incoming parts and materials may need to be considered based on historical data.
|Example of Process Failure Mode and Effect Analysis|
Maintaining Process Failure Mode Effect Analysis (PFMEA)
The PFMEA is a living document and should be reviewed whenever there is a product or process design change and updated, as required. Recommended actions updates should be included into in subsequent PFMEA along with the ﬁnal results. Another element of on-going maintenance of PFMEA should include a periodic review. Speciﬁc focus should be given to Occurrence and Detection rankings. This is particularly important where there have been product or process changes or improvements in process controls. Additionally, in cases when, either ﬁeld issues or production issues, such as disruptions, have occurred. the rankings should be revised accordingly.
Leveraging Process Failure Mode Effect Analysis (PFMEA)
If a new project or application is functionally similar to the existing product, a single PFMEA may be used with customer concurrence. Using a fundamentally sound baseline PFMEA as the starting point provides the greatest opportunity to leverage past experience and knowledge. If there are slight differences, the team should identify and focus on the efforts of these differences.
Linkage of Process Failure Mode Effect Analysis (PFMEA)
The DFMEA is not a “stand-alone” document.
In the development of a PFMEA it is important to utilize the information and knowledge gained in the creation of the DFMEA, However. the link between the two documents is not always obvious. The difﬁculty occurs because the focus of each FMEA is different. The DFMEA focuses on part function whereas the PFMEA focus on the manufacturing steps or process. Information in the columns of each form is not directly aligned. For example. Item/Function-Design does not equal Process Functions/Requirements; design failure mode do not equal process failure mode; potential design cause of failure does not equal potential process cause of failure. However, by comparing the overall analysis of design and process, a connection can be made. One such connection is between the characteristics identiﬁed during the DFMEA and PFMEA analysis. Another connection is the relationship between potential design cause of failure (DFMEA) and potential process failure mode (PFMEA). For example. the design of a feature such as 3 hole can cause a particular failure mode. The corresponding process failure mode is the inability of the process to manufacture the some feature as designed. In this example, the potential design cause of failure (hole diameter designed too large) would appear to be similar to the potential process failure mode (hole drilled too large). The potential effect of the failure mode for both design and process may be identical if there were no additional process related effects. In other words, the and result (effect) of the failure mode is the same. but there are two distinct causes. While developing the PFMEA, it is the team’s responsibility to ensure that all process related potential failure modes which lead to product related effects are consistent between the DFMBA and the PFMEA.
In addition to the list of Recommended Actions and their subsequent follow-up as a result of the PFMEA activity, a Control Plan should be developed. Some organizations may elect not to speciﬁcally identify the related product and process characteristics in the PFMEA. In this situation, the “Product Characteristics” portion of the Control Plan may be derived from the “Requirements” portion of the “Process Function/Requirements” column and the “Process
Characteristics” portion may be derived from the “Potential Cause(s) of Failure Mode” column. When the team develops the Control Plan, they need to assure that the PFMEA current controls are consistent with the control methods speciﬁed in the Control Plan.
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